The Macrophage-Stimulating Protein Receptor, hereinafter also referred to as “RON”, belongs to the c-met family of receptor tyrosine kinases. RON is a heterodimeric protein comprised of an extracellular alpha chain and a transmembrane beta chain. RON is first expressed as a single chain precursor, followed by cleavage into the alpha and the beta chains (1). It is believed that the beta chain is required for binding of its ligand, Macrophage-Stimulating Protein (“MSP”; a/k/a HGF-like protein), to the receptor, and the Kringle domains of 2 and 3 of MSP, are required for RON/MSP interaction. See, U.S. Publ. No. 2003/0073656. The extracellular domain of RON is thought to have little homology with the corresponding domains of the c-met family receptors. Indeed, binding of Hepatocyte Growth Factor (“HGF”), which stimulates other receptors in the c-met family, to the RON receptor, does not stimulate tyrosine kinase activity (WO 02/083,047).
RON is thought to have a role in cell migration, shape change and invasion of tissues by tumors (1). An earlier publication, however, reported a limited role for RON for inducing transformation, but described a promotion of invasive growth by RON activation (16).
Mutations, deletions, gene rearrangements and alternative mRNA splicing may cause activation of RON without any ligand binding (1). Variations in the tyrosine kinase domain of RON may play an important role in activation of RON (1). Cloning of RON from various cancer cell lines has shown RON activation due to various defects in the mRNA encoding for RON.
MSP is a member of the kringle-domain plasminogen-related protein family (1). As its name implies, MSP was originally found to stimulate macrophages by a variety of means (for review, see 2, 3). For example, addition of MSP to certain RON-expressing macrophages induced shape changes, chemotaxis, macropinocytosis, phagocytosis and immune mediator production (4, 5, 6). RON was also found to be expressed in epithelial cells such as keratinocytes where MSP was shown to phosphorylate RON and activate a number of signaling pathways that elicited cell adhesion/motility, anti-apoptotic and proliferative responses (7,8). Within the last few years, over-expression of RON has been observed in several epithelial tumors and cell lines (ex. colon (9, 10, 11), lung (12), breast (13)). In a recent study, lung tumors developed in transgenic mice engineered to over-express RON in their lungs (14, 15).
RON is expressed in a variety of human cancer cell lines. An antibody to RON can inhibit activation of the receptor (phosphorylated RON) as well as activation of the down-stream signaling molecules: phosphor-MAPK and phosphor-AKT in many of these cancer cell lines. Both of the anti-RON antibodies (RON6 and RON8) exemplified hereinbelow, are shown to significantly retard the ability of several cancer derived cell lines (HT-29 colon, H-292 lung, BxPC3 pancreas, JIMT-1 breast) to form tumors when injected into nude mice. This confirms that inhibition of RON receptor tyrosine kinase negatively influences the proliferation of these cancer cells, and underscores the utility of inhibiting RON in, for example, colon, lung, pancreatic and breast cancers. Using conventional Western blot and flow cytometry procedures, RON has been shown to be expressed in many human cell lines derived from a variety of cancers: colon (HT-29, Colo205, HCT-116, DLD-1, Sw480, Sw620), pancreatic (BXPC-3, CAPAN-2, ASPC-1, HPAF-II, L3.7p1#7, Hs766T), prostate (DU-145, PC-3), stomach (AGS, NCI-N87), lung (A549, H596) liver (HepG2, SNU-182) and breast (JIMT1, DU4475, AU565).
There is a continuing need in the art for developing treatments for various diseases, particularly cancer, based on identifying RON targets, including specific RON epitopes.